Why use _ for emphasized text?

[one0rb]

I'm curious to know why the decision was made to use _ as the delimiter for emphasized text?

Personally I think that it would be more intuitive and have more meaning if _ was used as the delimiter for subscript, as it often is in math-based markup like asciimath. That would make the delimiters for superscript and subscript be ^ and _ which nicely pair with text going 'up' and 'down'.

Furthermore, an easy way to implement this would be to just swap the _ and ~ delimiters around, I think?

[jgm]

Subscript in regular text (as opposed to math) is fairly rare, compared to emphasis, and there is a long tradition of using _ for emphasis, as well as an intuitive association with underlining.
Subscripts in (LaTeX) math are handled separately, e.g. $x_n = 3$.

[one0rb]

I see. Never thought about the association with underlining but that makes a lot more sense now. Thank you for the speedy clarification and information!